1. Field of the Invention
The present to geophysical exploration, and more particularly to seismic surveying and processing of seismic data to improve the quality of the large sets of data for more accurate results regarding subsurface formations.
2. Description of the Related Art
In geophysical exploration, seismic surveys are performed to produce images of the various rock formations in the earth. The seismic surveys obtain seismic data indicating the response of the rock formations to the travel of elastic wave seismic energy. The seismic data are then processed and analyzed to produce the images in both time and depth domains of the formations and their locations in an area of interest beneath the earth's surface. It is important for the processing results to be accurate. One of the bases for accurate processing results is that an accurate estimate of the velocity of travel of the seismic energy waves is recovered. However, the velocity function is not a fixed value over its paths of travel through the earth. The seismic velocity varies for several factors, such as the density of the rock in the formations and the depth of the formations during such travel.
During seismic surveys, the shallow or near surface part of the earth is where most of the complexities are concentrated, due to the existence of phenomena such as weathering and geomorphological processes. Geological features such as faults which generate sharp velocity contrasts in this section of the earth add to the complexity of the problem.
Accurate description of the variable velocities occurring in the near surface earth is very important for seismic imaging in the time and depth domains. Recent developments in the field of seismic velocity estimation by a process known as full waveform inversion (or FWI) has placed emphasis on the use of refracted energy arrivals present in the seismic data for long offset distances between the source and seismic sensors. The refracted energy arrivals are used to achieve maximum depth of penetration in velocity reconstruction, both in land and marine datasets. Seismic processing in either the time or depth domain is also becoming increasingly dependent on an insightful use of refracted waves at variable amounts of offsets corresponding to increasing depths of penetration.
Near surface velocity variations can distort the seismic images both in time and depth domains leading to erroneous evaluations of position, depth and nature of drilling targets. To correct for this, an estimate of near surface seismic wave velocities must be made. A common practice in land seismic data, and increasingly used also in long offset marine datasets, is to pick the first arrival time of direct and refracted waves on collections of seismic traces, known as gathers. The travel times of seismic waves through the earth from sources to receivers are then inverted to obtain estimates of seismic velocities extending at depths proportional to the source-receiver offsets and depending on the specific geology. The “first break picking” is a time consuming process, it requires human intervention to perform quality control and is considered a major bottleneck in the seismic data processing cycle.
Seismic industry practices in imaging and inversion (such as full wave inversion) have been rapidly evolving in recent years. However, the industry practices have, so far as is known, still involved processes which rely on some preliminary labor-intensive analytical work to select or pick from the data phenomena which are known as first breaks (or FB). Selection of an accurate and proper first break is necessary to obtain a robust estimate of the earth's acoustic velocity in the near surface region. The estimate of earth's acoustic velocity in the near surface region serves as the basis for subsequent processing to determine an accurate velocity function indicating the various acoustic velocities for the deeper earth formations as functions of time or depth.
There have been some automated or computerized autopicker methods developed and available over the years. However, these still have required significant manual intervention by an analyst for guiding the estimates, such as manual picking on a reduced set of data gathers to be used as a guide for the autopicker methods, as well as for quality control of the results from the autopicker process.
Further, post-picking quality control of the first break picks, whether from manual picks or from autopickers is largely unmanaged. So far as is known, no reliable methodology exists for handling billions of picks from large three dimensional seismic surveys. Thus, most of the work has taken the form of random checks of very small portions of the survey dataset. Large undetected errors are likely to remain in the data used for subsequent inversion processing. These errors can be due to noise or to mis-picking as a result, for example, of high-energy later arrivals from cycle skipping common in layered geology exhibiting velocity inversions between adjacent formation layers.